Green’s Dictionary of Slang

1. to join a criminal gang.

1904		Number 1500 Life In Sing Sing 261: I did the grand to Chicago and filled in with a yeg mob.
1924		G. Henderson Keys to Crookdom 404: Fill in. To join party.
1949		Monteleone Criminal Sl. (rev. edn).
1950		Goldin et al. DAUL 68/2: Fill. To agree to join a gang of which one is not a regular member in a specific criminal action.

2. to recruit into a criminal gang.

1937

(con. 1905–25) E.H. Sutherland Professional Thief (1956) 22: When he beat the murder rap, he was filled in by a shake mob.